converting folders of audio to high quality and smaller size for mp3 player

[sxizofenia]

hi everyone! i was recently using fre:ac to rip cds and i thought to use it to convert my music to my mp3 player. however, current folders/playlist contain more than 10.000 songs so i want to minimize the size but keep the quality as good as possible in order to be able to add more things in time. i tried to convert them in opus format but instead of putting all the songs together in a folder each song was put in one separate folder. How can i convert them fast without this happening?